Other mistake: Emmett states that the Cullens "haven't hunted in weeks", meaning all of the vampires should have black eyes. However, all of the actors still have their golden contacts in. This is especially noticeable on the Blu-Ray.
Other mistake: When Edward and Bella are playing chess on their Isle Esme, Edward takes one of Bella's pieces but puts his own piece down on the wrong square.
Answer: He's merely justifying why Bianca should be killed. While reading the note, he notes that she misspelled "Carlisle", then surmises she'll also make grammar errors. Aro does not tolerate imperfections, has little regard for human life, and most likely intended to dispose of her.
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